1. Field of the Invention
The present invention relates to focus adjustment mechanisms of phase difference type focus detection apparatuses that are particularly suitable for use in single-lens reflex ("SLR") cameras and the like.
2. Description of the Related Art
A phase difference type focus detection block integrated into a single-lens reflex camera is known and an example is explained with reference to FIG. 1.
In FIG. 1, between a taking lens 1 and a film surface 31, a quick return mirror 20 having a semi-transparent area is centrally located. Luminous flux taken in from taking lens 1 is reflected upward at quick return mirror 20 which, in turn, is directed towards a finder system. The finder system is comprised of a focusing screen 22, a condenser lens 23, a pentagonal prism 24, and an eyepiece 25. The optical and reflective qualities of the finder system will be apparent to those skilled in the art. On the other hand, luminous flux taken in from taking lens 1 which penetrates through the semi-transparent area of quick return mirror 20 is reflected at a sub-mirror 21 and is directed towards a phase difference type focus detection block 11. Field mask 2, field lens 3, aperture mask 4, reimaging lenses 5a1 and 5a2, line sensor 6, and reflecting mirror 7 are fixed in integrated unit 10 to make up the focus detection block 11.
To obtain accurate subject distance, the following two adjustments are made in phase difference type focus detection block 11. The two adjustments refer to: (1) adjustment of the optical axis of the focus detection block and that of the camera body (hereinafter referred to as "pupil exit adjustment"), and (2) rotational alignment around an optical axis for the reimaging lens and a pair of line sensors that receive an image (hereinafter referred to as "squint adjustment").
Focus detection block 11 is mounted onto a camera body using three screws and springs to prevent diversion by changing the entire angle of focus detection block 11 through an adjustment of the screws. Alternatively, as described in Japanese Patent Publication No. 62-161111, it is also possible to make a pupil exit adjustment by moving field lens 3 on the plane area that is perpendicular to the luminous axis of field lens 3.
Configuration of a typical phase difference type focus detection optical system is shown in FIG. 2. Field mask 2 restricts unnecessary light outside of the focus detection field, while field lens 3 conjugates a pair of openings 4a and 4b of mask 4 and the exit pupil of taking lens 1. Moreover, a pair of reimaging lenses 5a1 and 5a2 reimages on line sensor 6 the luminous flux that penetrates separate areas of the exit pupil of taking lens 1. Correlation for locations of two images that have been reimaged are obtained from the output of two photoreceptors 6a1 and 6a2 of line sensor 6 to thereby allow focus discrepancy to be calculated.
In FIG. 2, the pupil exit adjustment refers to the alignment of an optical axis that is determined in the area from field mask 2 to reimaging lenses 5a1 and 5a2 (i.e. the optical axis of a focus detection block) with the optical axis of taking lens 1 on the camera side. If this alignment is inadequate, openings 4a and 4b of aperture mask 4 are not projected to the proper location on the exit pupil of taking lens 1, and the luminous flux, that otherwise should enter reimaging lenses 5a1 and 5a2, is diverted at the exit pupil of taking lens 1, thereby affecting the precision of the focus detection procedure.
Squint adjustment refers to the rotational alignment of reimaging lenses 5a1 and 5a2 and line sensor 6 around an optical axis. If this alignment is inadequate, for example, if line sensor 6 is tilted away from the proper position of the plane area that is perpendicular to the luminous axis, photoreceptors 6a1 and 6a2 become tilted as shown in FIG. 10A, resulting in a vertical shift, thus detecting the focus points for subject images at different locations. Furthermore, if the plane areas of reimaging lenses 5a1 and 5a2 are tilted perpendicular to the luminous axis, projected images 30a1 and 30a2 are projected onto different locations; therefore, focus points for different locations of the subject images are detected as shown in FIG. 10B even if line sensor 6 is not tilted. Consequently, if a cross-eye adjustment is insufficient, photoreceptors 6a1 and 6a2 perform the photometric process for different locations of the subject images, and accurate focus detection can not be achieved with prior known devices of the type just described.
If line sensor 6 is rotated from the state shown in FIG. 10A so that the rotational positions thereof match with reimaging lenses 5a1 and 5a2 around the optical axis, this results in a state shown in FIG. 11A. If line sensor 6 is rotated from the state shown in FIG. 10B so that the rotational positions thereof match with reimaging lenses 5a1 and 5a2 around the optical axis, the resulting state is depicted in FIG. 11B. In both of the aforementioned cases, photoreceptors 6a1 and 6a2 perform photometry for the same location of a subject image. In FIG. 1, line sensor 6 is mounted on unit 10 by aligning it with reimaging lenses 5a1 and 5a2 around the optical axis.
In contrast to the structure for a focus detection apparatus for a camera shown in FIG. 1, a focus detection apparatus of a camera that is capable of providing detection focuses for multiple areas in an image plane is suggested in Japanese Patent Publication No. 63-11906. FIG. 3 depicts an example of such an apparatus wherein three focus points are detected. The basic configuration of the optical system of FIG. 3 is substantially the same as the structure depicted in FIG. 1.
Referring now to FIG. 3, field mask 12 comprises three openings 12a, 12b, and 12c. Opening 12a of the field mask is horizontal, whereas openings 12b and 12c are vertical. Field lens 13 comprises three lens parts 13a, 13b, and 13c. Additionally, aperture mask 14 comprises a pair of three aperture openings (indicated by dotted lines).
Reimaging lens 15 comprises three pairs of imaging lenses (15a1, 15a2), (15b1, 15b2), and (15c1, 15c2), and line sensor 16 has three pairs of photoreceptors (16a1, 16a2), (16b1, 16b2), and (16c1, 16c2). The combinations of lenses and photoreceptors correspond to three focus areas. Field mask 12, field lenses 13a, 13b, and 13c, aperture masks 14a, 14b, and 14c, reimaging lenses 15a, 15b, and 15c, and line sensors 16a, 16b, and 16c constitute an independent optical system that detects a focus point for each area. In this case, focus detection block 90 is secured to the camera body or the like (not shown) wherein field mask 12, field lens 13b, aperture mask 14, reimaging lens 15, line sensor 16, and mirror 7 are integrated as a unit.
The above-illustrated structure depicted in FIG. 3 is such that if focus detection block 90 is mounted on a camera body using screws and springs, thereby changing the entire angle of focus detection block 90 with an adjustment of the screws to perform a pupil exit adjustment for one of the areas, the other two areas cannot be adjusted. As described in Japanese Patent Publication No. 62-161111, if the field lens 13 is moved so that its plane area is perpendicular to its optical access for a particular area, such an arrangement provide the same results as noted above. Additionally, when the rotational locations of the photoreceptors/photosensors of line sensor 16 and reimaging lens 15 are aligned for a squint adjustment for one area, the other two areas cannot be adjusted.
The present invention solves the aforementioned problems. More particularly, the present invention provides a focus detection apparatus wherein pupil exit and squint adjustments may be independently achieved for multiple focus detection areas with great accuracy and minimal space requirements.